The present application provides a non-aqueous electrolytic solution secondary battery having an excellent life characteristic.
In recent years, the study of rechargeable secondary batteries is being advanced as a power source which can be used over a long period of time conveniently and economically with tremendous progress of a variety of electronic instruments. As representative secondary batteries, there are known a lead storage battery, an alkaline storage battery and a non-aqueous electrolytic solution secondary battery.
Of the foregoing secondary batteries, in particular, a lithium ion secondary battery which is a non-aqueous electrolytic solution secondary battery has advantages such as high output and high energy density. The lithium ion secondary battery is configured of at least a positive electrode and a negative electrode each having an active material capable of reversibly inserting and extracting a lithium ion and a non-aqueous electrolytic solution.
The lithium ion secondary battery is widely used for mobile instruments such as a notebook type personal computer, a mobile phone and a camcorder in view of characteristic features including light weight and high energy density. In lithium ion secondary batteries which are generally put into practical use at present, lithium/cobalt composite oxide LiCoO2 having a stratified rock salt structure is used for a positive electrode active material. However, cobalt is poor in natural resources and expensive, and therefore, a positive electrode active material which can be a replacement thereof is groped. Of these, the development of a positive electrode material on a basis of manganese Mn which is rich in natural resources and cheap is desired as a replacement of Co. There is proposed lithium/manganese composite oxide LiMn2O4 having a spinel structure and having a space group Fd3m as the positive electrode material on a basis of Mn. This LiMn2O4 has a high potential as 4 V class vs. lithium potential, the value of which is equal to LiCoO2. Furthermore, LiMn2O4 is a very promising material in view of the matters that it is easily synthesized and has a high battery capacity and is already put into practical use.
However, in fact, the lithium ion secondary battery configured of LiMn2O4 as a positive electrode active material involves a problem that Mn is dissolved in an electrolytic solution during the cycle to cause cycle deterioration and a problem that the stability is insufficient in view of characteristics.
Now, a variety of studies regarding a material based of iron Fe as the positive electrode material have been made. Fe is a material which is richer in natural resources and cheaper than Mn, and if a positive electrode material on a basis of Fe can be realized, such is more favorable. As the positive electrode material on a basis of Fe, studies are made centering on, for example, a material composed of, as a basic composition, LiFeO2 which has a structure similar to LiCoO2 or LiNiO2. However, LiFeO2 is difficult in preparation and instable in structure, and its sufficient characteristics as a positive electrode active material for lithium ion secondary battery have not been realized yet.
On the other hand, there is proposed the use of lithium iron phosphate LiFePO4 as a positive electrode material of lithium ion secondary battery, see, for example, JP-A-9-171827. LiFePO4 has a large volume density as 3.6 g/cm3, generates a high potential of 3.4 V and has a large theoretical capacity as 170 mAh/g. Moreover, since LiFePO4 contains one Li capable of being electrochemically dedoped in the initial state per Fe atom, it may be said that LiFePO4 is a promising material as the positive electrode active material of lithium ion secondary battery.